1. Technical Field
The present invention relates to a vehicle occupant protection system. In particular, the present invention relates to a vehicle seat belt system having an electric motor driven pretensioner.
2. Description of the Prior Art
A typical vehicle seat belt system includes a length of seat belt webbing wound on a spool of a seat belt webbing retractor. The seat belt webbing is extensible about a vehicle occupant for helping to protect the occupant in the event of a vehicle crash condition. The spool rotates in a belt withdrawal direction as the occupant withdraws seat belt webbing from the retractor. A rewind spring is connected with the spool and biases the spool for rotation in an opposite belt retraction direction.
It is known to use a pretensioner to remove slack from seat belt webbing in the event of a vehicle crash condition. Typically, a pretensioner includes a pyrotechnic device that is actuated when a crash condition is sensed. After actuation of the pyrotechnic device, the pretensioner must be replaced.
Some seat belt systems use an electric motor to pretension the belt webbing. If an electric motor is used to pretension belt webbing, the motor can tend to overheat if it remains energized in a stall condition maintaining tension on the belt webbing.
The present invention is an apparatus for helping to protect an occupant of a vehicle seat. The apparatus comprises a seat belt and a seat belt retractor including a spool supported for rotation about a spool axis in a seat belt withdrawal direction and an opposite seat belt retraction direction. The apparatus includes a pretensioner energizable to rotate the spool in the seat belt retraction direction to apply a predetermined amount of tension to the seat belt. The apparatus also includes a first mechanism to maintain the predetermined amount of tension on the seat belt after the pretensioner applies the predetermined amount of tension to the seat belt. The apparatus further includes a second mechanism to block rotation of the spool in the seat belt withdrawal direction. The second mechanism is actuatable if the tension on the seat belt exceeds the predetermined amount.
In a preferred embodiment, the apparatus includes an electric motor energizable to rotate the spool in the seat belt retraction direction and de-energizable. The electric motor is energizable to rotate the spool in the seat belt retraction direction to apply a predetermined amount of tension to the seat belt. The apparatus also includes an electrically actuatable brake associated with the motor. The brake is actuatable to maintain the predetermined amount of tension on the seat belt after the motor applies the predetermined amount of tension to the seat belt.